Fluval U2 Underwater Filter – ClearMax Cartridge (2-Pack) – A two-pack of ClearMax filter cartridges for the Fluval U2 internal filter. ClearMax is a special media that removes phosphate, nitrite, and nitrate from aquarium water, thereby reducing algae growth and improving water quality. In the U2 filter, these cartridges slide into place for easy maintenance. Regular use of ClearMax in your Fluval U2 helps maintain low nutrient levels, resulting in clearer water and a healthier environment for fish and plants.